This application claims the priority benefit of Taiwan application serial no. 89120195, filed Sep. 29, 2000.
Field of the Invention
The invention relates in general to a method of improving photoresist profile. More particularly, this invention relates to a method of using a high temperature flow bake to make the photoresist layer reflow, so as to improve the profile thereof. Description of the Related Art
In the conventional method for patterning or defining a contact window or a via hole, a photoresist layer is formed on a surface of the layer in which the contact window or the via hole is to be formed. An opening is formed by patterning the photoresist layer to expose the portion at which the contact window or the via hole is formed subsequently. The conventional method to patterning photoresist layer, as shown in FIG. 1, comprises the steps of photoresist coating (Step 101), soft bake (Step 102), exposure (Step 103), post-exposure bake (Step 104), development (Step 105) and the hard bake (Step 106).
After the steps of exposure 103, the post-exposure bake 104, and the development step 105, a hard bake step 106 is performed to evaporate the remaining solvent contained in the photoresist layer. In addition to minimize the containing solvent, the adhesion of the photoresist layer can also be improved. Furthermore, the hard bake step is also advantageous to resist of the subsequent etching and ion implantation processes. The selectivity of such processes can be increased to etch or implant selectively only the positions uncovered by the photoresist layer.
A hot plate is typically used for performing the hard bake step. In the conventional method, as shown in FIG. 1, the temperature for the hard bake 106 is about 110xc2x0 C. for about 90 seconds. The purpose is to solidify the photoresist and to form the profile of the photoresist layer. Therefore, an operation temperature of the bake hard step is normally less than the glass transition temperature (Tg) of the photoresist layer.
After performing the hard bake step, an after develop inspection (ADI) is performed to ensure whether the photoreist layer meets the specification. A top down inspection by scanning electron microscope (SEM) is commonly used for the after develop inspection.
FIG. 2 shows a cross sectional view of the SEM top down inspection result for a photoresist layer being exposed when defocus occurs. A photoreist profile being performed with the steps of exposure, post exposure bake and the hard bake is shown. As shown in FIG. 2, at a top corner of the opening in the photoresist layer, a top reentrant is formed. A narrower opening observed by the SEM top down inspection thus very likely to fail the specification, so that a rework is required. However, if observing from the cross sectional view as shown in FIG. 2, only the top portion of the opening is narrowed, while the lower portion of the opening can meet the specification.
The invention provides a method of improving the photoresist profile. A photoresist layer is formed, followed by performing the steps of soft bake, exposure, post exposure bake, development and hard bake with an operation temperature higher than the glass transition temperature of the photoreist layer.
The invention further provides another method of improving the photoresist profile. A photoresist layer is formed, followed by performing the steps of soft bake, exposure, post exposure bake, development, hard bake and flow bake. The hard bake is performed at a temperature lower than the glass transition temperature of the photoreist layer, while the flow bake is performed at a temperature higher than the glass transition temperature of the photoresist layer.
By the method provided in the invention, the reentrant formed in the conventional method can be eliminated by re-flow of the photoresist layer. The processing window is widened and the mask error factor is also reduced. As a result, the uniformity of critical dimension (CD) can thus be effectively enhanced.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.